Digital time delay integration (TDI) scanning array devices have been developed in recent years for applications in broad band (panchromatic) and multispectral imaging (MSI). These devices are typically constructed as 2-layer hybrid focal planes, comprised of a photo-detector layer and a Si-CMOS read-out integrated circuit (ROIC) that is bonded to the photo-detector layer. The ROIC performs a digital domain TDI function using high speed memory accumulators and shift registers that are configured to emulate an analog high speed CCD capacitor chain used in conventional analog TDI detectors. For conventional multispectral imaging applications, which support “m” spectral channels, the focal plane array is typically configured with “m” independent TDI arrays, each separated by a finite “in-track” spatial offset and with corresponding “m” optical band pass filters to selectively pass only the spectral band of interest.
An example of a conventional multispectral imager 10 with four spectral bands (m=4) is shown in FIG. 1. In this example, four independent TDI arrays are required 11, 12, 13 and 14 in order to image a scene with four different color bands. Each array is covered by a discrete color filter (not shown). Each array is also independently clocked and read out, as the image scene moves relative to each array at a constant velocity.
Conventional TDI scanning is used to provide long integration times on a moving platform without blurring the image. This is achieved by taking a series of “n” high speed snapshots (or frames), each of which instantaneously freezes the platform motion, and subsequently shifting and adding the signal for each successive pixel in a time sequence from frame n to frame n+1, in a manner that tracks the platform velocity. This technique allows for arbitrarily long integration times on a moving platform that is limited by the number of TDI channels in the array (typically between 24-128 channels).
The in-track spatial offset between arrays, as shown in FIG. 1, can produce significant parallax error in remote sensing systems after aligning the pixels from the four bands to compensate for the offsets in the final image. For large arrays the parallax error between the first and last filter band can make the imagery unusable. The off-chip digital readout circuitry must also be repeated for each band.
The temporal offset between the first and last band is as follows:Offset=(array width+offset between arrays)*n arrays*Tclk                 where n is the number of arrays, Tclk is the pixel clock for the TDI shift register, and        array width=max TDI*Pixel pitch        
Accordingly, the more numerous the number of arrays, the greater the temporal offset between the first and last band. The greater the offset, the larger is the parallax error.
FIG. 8 shows an alternate method for achieving multispectral images using a filter wheel with a single TDI detector. In this example, there are 6 color bands (m=6) that are acquired by sequentially integrating the TDI detector with each color in the color wheel then switching to the next color. This results in reduced parallax offset as the offset between arrays is now eliminated.
In conventional analog TDI scanning detectors (e.g. CCD's), the TDI integration is done by transferring and accumulating the charge along each cascade (for example, each row) in the array in-phase with the image motion to build up photo electrons. Once the image reaches the end of the cascade, photo electrons are dumped onto an output node and converted into digital values.
The digital TDI system accumulates signal electrons by frame stepping where each frame is completely readout to the accumulator and summed with the next frame in the sequence. In the digital TDI domain, the integration occurs in the accumulator, after the photo charge has been detected and converted to a digital value. An example of digital accumulation, and pixel-by-pixel summation (integration of photo electrons in the analog domain), which is in-phase with the image motion is shown in FIG. 2.
As shown in FIG. 2, the scan direction of the imager is from right to left in the x-direction, and time is sequenced in the y-direction. The image array includes one row of six detectors (A-F) oriented along the “in-track” or x-direction. Thus, a snap, or frame image consists of six pixels (a detector may be referred to herein as a pixel, once a digital value has been assigned to the analog voltage outputted by the detector). Each frame 21 in the example is shifted by one pixel, which is less than the array size of six detectors. The number of pixels shifted may be determined by a system controller (not shown), which may be external to the CMOS ROIC; the amount of shift may be based on the relative speed of the imager with respect to the scene under observation. The number of pixels shifted may be greater than one pixel, and may be dynamically controlled by the system controller.
Each frame 21 is completely read out to an accumulator and summed with the next frame 21 in an accumulation sequence, designated as 22. Thus, pixel B of frame 1 and pixel A of frame 2 are summed in the sequence. Next, pixel A of frame 3 is added to pixels C and B of respective frames 1 and 2. By the time the imager has moved to frame 6, the accumulation sequence includes the sum of pixels A-F from frames 1-6, respectively. Accordingly, the first column of full TDI accumulation occurs after frame 6 is snapped and summed by the imager.
The example of FIG. 2 depicts a digital TDI sequence showing accumulation for a single color band. A TDI walk-on for the single band is 5 frames and a full TDI occurs at 6 frames. A TDI walk-off, which is similar to the TDI walk-on, is also 5 frames, as shown. In general, however, if more than one band is desired, the number of columns required is as follows:Columns=Bands×TDI
Accordingly, if the required TDI is 6 and the number of color bands required are 4 (as shown in the example of FIG. 1), the number of necessary columns become 24 (four accumulators needed). With four bands, a width of 24 columns is required. In the example of FIG. 1, the in-track spatial offset between arrays would produce significant parallax error in remote sensing systems, because the pixels in the final image require spatial alignment. For larger color arrays, the parallax error between the first and last color band can make the imagery unusable.
As will be explained, the present invention overcomes the aforementioned parallax error by providing multiple register banks which have the capability to accumulate multiple interlaced images, while only using a single accumulation ROIC per detector array.